This invention relates to marine pipelaying using a pipelaying vessel to lay an offshore pipeline. The invention relates particularly to pipeline accessory structures such as pipeline end terminations and in-line tee assemblies, and to techniques for providing foundations for such accessories on the seabed.
Marine pipelaying techniques that involve fabrication of a rigid pipeline on a vessel are generally categorised as either S-lay or J-lay, although variants and hybrids of those techniques have been proposed and used. The invention may be used in S-lay or J-lay operations and also in reel-lay operations.
The S-lay technique, for example, involves welding together successive pipe joints at a series of working stations in a generally horizontal firing line on the deck of a pipelaying vessel, from which the pipeline is launched into the water over a stinger. A series of tensioners grip the pipe to control its movement relative to the vessel under the load of the pipe catenary extending between the vessel and the seabed.
S-lay is often preferred to J-lay for its inherently greater lay rate. This has led to the development of a variant of S-lay known as ‘Steep S-lay’, which is adapted for deep- and ultradeep-water applications. As its name suggests, Steep S-lay involves setting the lift-off point of the pipe from the stinger close to vertical.
References in this specification to S-lay are intended to encompass Steep S-lay where the context allows. A Steep S-lay operation will be used in this specification to exemplify the invention.
Pipelines are routinely fitted with accessories during fabrication to provide operational flexibility, to create desired field layouts and to support future field extensions. Such accessories may be disposed at the ends of the pipeline and within it. They include in-line tee assemblies (ILTs or ITAs), pipeline end manifolds (PLEMs), pipeline end terminations (PLETs, sometimes called flowline end terminations or FLETs), tie-in branches, wyes, tees, shutdown valves, pigging connections, pig-launching and pig-receiving equipment and other subsea structures.
Whilst the advantages of the invention are particularly apparent with heavier accessories such as PLETs, the invention will be exemplified in this specification with reference to an in-line tee or ILT. An ILT is a transition device that is used on pipelines and flowlines carrying production oil/gas or water injection fluids. It is a subsea hub for connection to another system, which may be a manifold, a wellhead or a PLET. An ILT is installed in-line with a pipe such as a flowline pipe; it may, for example, effect a connection between the flowline and a wellhead via a subsea jumper or spool.
The main functional parts of an ILT are a connector for making the required connection and a valve to control the flow through the connector. These parts are supported by a frame. The frame may support more than one connector and more than one valve. The total weight of an ILT is typically several tons. Some accessories such as PLETs may weigh tens or even hundreds of tons.
Once on the seabed, the weight of an accessory such as an ILT must not be borne by the pipe that is attached to the accessory. The accessory must instead be supported by a foundation, especially when it is laid on a soft, muddy seabed as is common in deep- and ultradeep-water locations. While supporting the weight of the accessory and external loads, the foundation must keep the accessory and the attached pipes stable by resisting rotation and lateral movement and must also avoid excessive settlement into the seabed.
In deep- and ultradeep-water applications, foundations for pipeline accessories most commonly comprise mudmats. A mudmat typically comprises a horizontal top plate, which is usually rectangular. Mudmats are usually paired, one each side of the pipeline, and are typically 5 m to 10 m in width and 5 m to 20 m in length.
Mudmats may be adapted to slide across the seabed to allow for expansion and contraction of the pipe in use. Alternatively, mudmats may be adapted to dig in to the seabed to locate the mudmat against sliding across the seabed, in which case there may be provision for the supported accessory to move to a limited extent with respect to the mudmat. To resist sliding, the top plate of a mudmat may be provided with a vertical peripheral skirt extending to a depth of up to more than 1 m. There may also be vertical partition walls disposed under the top plate and intersecting the surrounding skirt to stiffen and support the top plate and to improve engagement with the seabed.
A mudmat foundation has to cope with the high centre of gravity of the accessory it supports and the torque that may be applied by jumpers or spools attached to the accessory when in position for use, while keeping the accessory and the pipe stable without becoming embedded in the seabed. In principle, therefore, mudmats should be as long and wide as possible to define a large base area.
However, there are limits on mudmat size imposed by considerations of cost, weight and ease of installation. In this respect, pipeline installation is not solely a pipelaying activity but also involves handling and lowering accessories attached to the pipe. Consequently, the overall speed of pipeline installation is not determined simply by the rate at which a pipelaying vessel can fabricate and lay pipe, but also by the ability of the vessel to install accessories as part of the pipeline.
To ease the integration of large accessories in pipeline installation operations, it has been proposed for pipeline accessories such as ILTs to be fitted with folding mudmats. Such mudmats pass with the accessory along the firing line of a pipelaying vessel and are overboarded in a compact folded configuration. They are then opened into a wider deployed configuration upon, or just before, reaching the seabed. An example of this approach is disclosed in WO 2012/101525.
The folding mudmat solution exemplified by WO 2012/101525 is a good way to exploit the available space on a pipelaying vessel. However, the maximum size of an accessory fitted with folding mudmats is still limited to the space that is available along the firing line of the vessel, meaning that the available mudmat area may not be sufficient to support a heavy accessory.
Another solution is to pass only a part of the accessory structure through open tensioners, grippers or clamps on the firing line of a pipelaying vessel, and then to assemble the full structure, including mudmats, after or downstream of those tensioners, grippers or clamps. However, in addition to limits on the size of structure that may pass along the firing line, the deck layout of the vessel may impose space constraints that hinder or preclude downstream assembly operations.
It is also known to bypass a restriction in the firing line of a pipelaying vessel by passing an accessory over the side of the vessel away from the laying axis and then recovering the accessory to the laying axis downstream of the restriction. This avoids mudmats fitted to the accessory having to pass through the restriction altogether; however, handling a bulky accessory in this way can be a complex and time-consuming operation.
If an accessory structure is launched into the sea with mudmats already fitted to it, the size and stiffness of the mudmats may not be compatible with the bending radius of the pipe on the stinger. This is a problem in S-lay operations in general but is a particular problem in Steep S-lay operations, where the radius of curvature of the stinger is small and the overbend strain experienced by the pipe is correspondingly large.
A solution to these problems is to assemble an accessory and a foundation on the seabed in separate installation operations. One approach to this is disclosed in WO 2011/110950, which is regarded by the inventors as the closest prior art although it actually teaches away from the invention as will be explained. Here, mudmat foundations are pre-installed on the seabed, for example by a crane, and pipeline accessories are docked with the foundations upon laying the pipeline across them. However, managing the operational risk of being unable to unite an accessory with a pre-installed foundation tends to increase the size and weight of the foundation structure. Pre-installation of foundations also creates planning constraints on siting the foundations and routing the pipeline.
Moving on to more distant background art, WO 2011/114170 and WO 2011/114172 disclose techniques for overhauling a flanged coupling of a pipeline. In those disclosures, a pipeline section containing the flanged coupling is lifted from the seabed onto a support frame positioned beside the pipeline. However, the pipeline section containing the flanged coupling needs no permanent foundation: the support frame acts only as a temporary foundation for the overhaul operation and is removed after the flanged coupling has been placed back on the seabed.
GB 2070187 discloses an adjustable foundation structure that is attached permanently to a pipe underwater. Once the structure is positioned on the seabed under the pipe, jacks raise the pipe to a desired height above the seabed. The structure has legs that are driven into the seabed by self-weight and by the weight of the pipe.
WO 94/08166, U.S. Pat. No. 4,147,455 and WO 93/01438 also disclose permanent vertically-adjustable pipe support structures that are installed after the pipe and clamped around it. In these instances, however, the structures are designed to mitigate free-span problems where a pipe spans a depression in the seabed.
The pipe-support structures of GB 2070187, WO 94/08166, U.S. Pat. No. 4,147,455 and WO 93/01438 support only a pipe and not an accessory structure with valves and connectors, which as noted above will typically add several tons to the weight of the pipe. Whilst the feet of these structures are provided with pads, they could not withstand the weight of an accessory without becoming embedded in the soft, muddy seabed that is characteristic of deep- and ultradeep-water locations. Also, these structures cannot be attached to or inserted under a pipe if that pipe is embedded in a soft seabed.
Of course, where pipe-support structures are attached to a pipe directly as in GB 2070187, WO 94/08166, U.S. Pat. No. 4,147,455 and WO 93/01438, it is possible to tolerate considerable inaccuracy in their longitudinal position along the pipeline. However, such a degree of inaccuracy is not permissible when uniting an accessory and a mudmat foundation on the seabed such as is described in WO 2011/110950 mentioned above.
US 2012/0263541 discloses a subsea supporting structure that elevates and supports a pipe above the seabed. The supporting structure can be installed before or after the pipe is laid, although installation under an already-laid pipe can only be done if the pipe already has a sufficient initial elevation above the seabed. Whilst the supporting structure has a mudmat foundation, there is no disclosure of a pipeline accessory: the supporting structure acts directly on the pipe.